To provide maximum service with minimal impact on useable space, it is known to provide more than one elevator car traveling independently within the same hoistway. In some systems, call assignments are extremely rudimentary so that there is no problem with potential collision of cars, but such systems do not add significant service since many calls cannot be assigned. Examples are illustrated in U.S. Pat. Nos. 5,419,414, 6,360,849 and U.S. Pat. No. 2003/0164267.
To maximize the benefit of having more than one car traveling in a single hoistway, additional controls are needed to assure the cars will not collide, while providing significant increases over the single-car service.